This invention relates to a means and method for simultaneously triggering serially connected SCR's and, more particularly, to circuitry for simultaneously triggering serially connected SCR's via a plurality of pulse transformers in a reliable and cost effective manner, in order to maintain equalization of voltage sharing among the SCR's.
When switching of an operating voltage in excess of the blocking voltage capabilities of a single SCR is desired or necessary, a series combination of SCR's can be employed if certain design considerations are observed. Such considerations include equalization of voltage sharing (both forward and reverse) between individual SCR's at steady state and transient operating conditions, external voltage equalization network and gating circuit design. Also, component parameter variations such as junction capacitances, delay times, forward voltage drops, reverse recovery time for individual SCR's (e.g. snubber circuits) must be taken into account.
Present means and methods used with circuits for triggering serially connected SCR's include techniques catagorized broadly as simultaneous triggering and slave triggering. In slave triggering for serially connected SCR's, the gate of a master SCR of the serially connected SCR's is first triggered, and as its forward blocking voltage begins to collapse a gate signal for triggering is applied to a slave or next serially connected SCR. As the forward blocking voltage of the slave SCR begins to collapse, a gate signal for triggering may be applied to a second slave SCR. This process may be repeated, thus providing sequential or staggered triggering of serially connected SCR's. The entire operating voltage must be supported or blocked by those serially connected SCR's which have not yet switched, which may cause the voltage across an individual SCR to exceed its rated peak forward voltage (PFV) and result in permanent damage to the SCR due to the cumulative time delay associated with triggering each successive SCR. Thus slave triggering is not desirable for greater than about 2 or 3 serially connected SCR's.
Simultaneous triggering of all gates of serially connected SCR's is generally preferred in order to prevent the voltage impressed across any individual SCR from exceeding the voltage rating of that SCR, even for a short period of time. However, when it is desired to trigger the gates of a plurality of serially connected SCR's, say generally greater than 2, it is difficult using present schemes, such as direct transformer triggering from a plurality of secondary coils (each coupled to the same primary of a pulse transformer), due to reflected impedance to the pulse transformer and high voltage isolation requirements of the pulse transformer, to provide a trigger pulse having adequate current and a rise time fast enough to ensure that all serially connected SCR's are indeed simultaneously triggered. If fewer than all serially connected SCR's are simultaneously triggered, then the entire operating voltage will be impressed across the SCR's which are not first triggered. This may cause the voltage impressed across a single SCR to exceed the voltage rating of the SCR and thus overstress the SCR, which may lead to permanent damage of the SCR. In the worst case, all but one of the serially connected SCR's is triggered. For example, assume there are five serially connected SCR's, each having a voltage rating of 1400 volts, and the series combination is blocking a voltage of 6000 volts. If it is attempted to switch all SCR's simultaneously, and one doesn't switch, then the 6000 volts will be impressed across the one which does not switch, causing it to fail. When the system next attempts to turn off (i.e. revert to the blocking state) the four remaining SCR's, the 6000 volts will be divided equally among the four SCR's. This results in a voltage of about 1500 volts across each SCR, which exceeds the voltage rating of each SCR and may cause one or more additional SCR's to fail.
In certain applications, it may be possible to use a light-fired SCR, i.e. an SCR having a gate responsive to light. However, light fired SCR's are expensive and may not be available generally. Further, a light source, such as a cesium lamp or laser LED, generally needed to ensure sufficient current to trigger a plurality of SCR's, is also expensive.
Accordingly, it is an object of the present invention to provide a means and method of triggering a plurality of serially connected SCR's which ensure simultaneous triggering of each SCR.
Another object of the present invention is to provide a means and method for triggering a plurality of serially connected SCR's using components which are readily available.
A further object of the present invention is to provide a means and method for triggering a plurality of series SCR's which are cost effective and relatively inexpensive.
Yet another object of the present invention is to provide a means and method for triggering a plurality of series SCR's wherein power to trigger each SCR comes from a relatively low voltage source and not from the relatively high operating voltage across the plurality of SCR's.